Methods for Manufacturing a Contact Grid on a Photovoltaic Cell

ABSTRACT

Processes for fabricating a contact grid for a photovoltaic cell generally includes providing a photovoltaic cell having an antireflective coating disposed on a sun facing side, the photovoltaic cell comprising a silicon substrate having a p-n junction; soft stamping a pattern of a UV sensitive photoresist and/or polymer onto the antireflective coating; exposing the UV sensitive photoresist and/or polymer to ultraviolet radiation to cure the UV sensitive photoresist and/or polymer; etching the pattern to form openings in the antireflective coating that define the contact grid; stripping the UV sensitive photoresist and/or polymer; and depositing a conductive metal into the openings defined by the pattern. The metal based paste can be aluminum based, which can be annealed at a relatively low temperature.

BACKGROUND

The invention relates to photovoltaic cells, and more particularly, tomethods for manufacturing a contact grid including bus bars on aphotovoltaic cell.

Current photovoltaic cells such as solar cells are covered with ametallic contact grid for transporting current and minimizing currentloses due to resistance through underlying silicon-containing layers.The patterns of conductive metal lines that define the metallic contactgrid and/or bus bars are generally screen printed using thick filmtechnology to layer a conductive paste of metal materials into a desiredpattern. The metal paste, typically silver, is dried and then fired,i.e., sintered, at relatively high temperatures. Problems related tocurrent processes include the costs associated with the use of silver,which is relatively high and that the solar efficiency of thephotovoltaic cell can be degraded during the annealing process becauseof the high temperatures currently used to effect sintering of thesilver paste. For example, once the silver paste is screen printed, theannealing step typically occurs at temperatures of about 900° C. for anextended period of time, typically about an hour or more, which cancause the degradation.

Accordingly, there is a need for improved manufacturing processes thatare less expensive and less prone to degradation.

BRIEF SUMMARY

The shortcomings of the prior art are overcome and additional advantagesare provided through the provision of manufacturing process thatutilizes a relatively low temperature firing and soft stamping processfor forming the contact grid and/or bus bars. In one embodiment, aprocess for fabricating a contact grid for a photovoltaic cell comprisesproviding a photovoltaic cell having an antireflective coating disposedon a sun facing side, the photovoltaic cell comprising a siliconsubstrate having a p-n junction; soft stamping a pattern of a UVsensitive photoresist and/or polymer onto the antireflective coating;exposing the UV sensitive photoresist and/or polymer to ultravioletradiation to cure the UV sensitive photoresist and/or polymer; etchingthe pattern to form openings in the antireflective coating that definethe contact grid; stripping the UV sensitive photoresist and/or polymer;and depositing a conductive metal into the openings defined by thepattern.

In another embodiment, a process for fabricating a contact grid for aphotovoltaic cell comprises providing a photovoltaic cell having anantireflective coating disposed on a sun facing side, the photovoltaiccell comprising a silicon substrate having a p-n junction; stamping apattern of a photoresist and/or polymer onto the antireflective coating;curing the photoresist and/or polymer; etching the pattern in theantireflective coating to form openings that define the contact grid;stripping the UV sensitive photoresist and/or polymer; stamping analuminum based metal paste into the openings of the pattern by dipping astamp into the aluminum based metal paste, wherein the stamp comprises aplurality of projections corresponding to the openings having a diameterand/or width smaller than the openings; stamping the projections intothe openings; and filling the openings with the metal paste; andannealing the aluminum based metal paste.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention. For a better understanding of the invention with advantagesand features, refer to the description and to the drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other objects, features, andadvantages of the invention are apparent from the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIGS. 1-7 depict an exemplary process for fabricating contact grids fora photovoltaic cell; and

FIG. 8 illustrates a flowchart describing an exemplary process forfabricating contact grids for a photovoltaic cell.

The detailed description explains the preferred embodiments of theinvention, together with advantages and features, by way of example withreference to the drawings.

DETAILED DESCRIPTION

Before describing the present invention in detail, it is to beunderstood that the terminology used herein is for the purpose ofdescribing particular embodiments only, and is not intended to belimiting. It must be noted that, as used in this specification and theappended claims, the singular forms “a,” “an,” and “the” include pluralreferents unless the context clearly dictates otherwise.

Disclosed herein are methods for forming the contacts including bus barswhere applicable onto photovoltaic cells at a stage in the manufacturingprocess subsequent to formation of the junctions, wherein thephotovoltaic cell includes an antireflective coating disposed over thejunction. The process is economical and provides lower annealingtemperatures, thereby substantially preventing solar efficiencydegradation.

Photovoltaic cells are generally formed on a silicon substrate 12,typically a p type boron doped substrate. The junctions can be formed bydiffusing an n type dopant 14 onto the p type silicon substrate viaphosphorous diffusion. One example of phosphorous diffusion includescoating phosphosilicate glass compounds onto the surface of the p-typesilicon substrate and subsequently annealing in a furnace to effectdiffusion. Once the p-n junction is formed, the antireflective coating16 is commonly provided on the front or sun facing side so as to reducereflection losses of photons. The resulting photovoltaic structure 10 atthis stage in the manufacturing process is generally shown in FIG. 1

As will be described in detail herein, the process for forming thecontacts including the bus bars includes the use of a soft stampingprocess. In the soft stamping process, a stamp master, which istypically fabricated from a quartz material, is employed and representsa negative of the desired contact pattern. A soft stamp is formed fromthe stamp master by coating/depositing a UV sensitive soft stampmaterial. As used herein, the term “soft” when describing a stamp or itsmaterial is a relative term which denotes a stamp or material that canmore easily deform around substrate features than a rigid stamp. In thismanner, the soft stamp can be used to secure accurate stamping on unevensurfaces. In one embodiment, the UV sensitive soft stamp material iscompliant and made of a soft elastomeric material. A non-limitingexample of a suitable stamp material is polydimethylsiloxane (PDMS),which is subsequently cured to form the soft stamp. Other suitablematerials are those with equivalent properties as the PMDS. Examplesinclude, without limitation, various urethanes and optical adhesives.The stamp master can be utilized to form multiple soft stamps as may bedesired. The resulting soft stamp itself can be cleaned with an aqueoussolution after each stamping process.

The softness of the stamp is generally dependent upon substratefeatures. For example, for most PDMS, a suitable material for a softstamp, the value of E (Young's modulus) is within the range of about 0.1to about 10 MPa, and the value of G (shear modulus) is less than orequal to about 1 MPa. On the other hand, for silicon, a suitablematerial for a rigid stamp, the value of E (Young's modulus) is equal toabout 130 GPa, and the value of G (shear modulus) is equal to about 30GPa. These values for E and G for a soft and rigid stamp are onlyrepresentative values, and they do not establish nor limit suitableranges for values of E and G for a soft and rigid stamp.

As shown in FIG. 2, a UV sensitive photoresist and/or polymer 18 is softstamped onto the surface of the antireflective coating 16 using thestamp. The stamp has the pattern of the contact grid including the busbars being used to contact the front surface of the substrate and iscoated with the UV sensitive photoresist and/or polymer. The patternedUV sensitive resist/polymer 18 is then transferred by the stamp onto theantireflective coating 16 and subsequently UV cured to form a UV curedphotoresist/polymer 20 as shown in FIG. 3.

In FIG. 4, openings 22 are then selectively etched into theantireflective coating 16 to the n-doped silicon layer 14 using the UVcured photoresist and/or polymer 20 pattern as a mask. The etchingprocess is not intended to be limited to any particular type and is wellwithin the skill of those in the art. For example, a phosphoric acidbased wet etching chemistry can be used that provides the desiredselectivity to form the openings extending through the antireflectivelayer to the underlying silicon layer.

After the openings 22 are formed, the UV cured photoresist and/orpolymer 20 is stripped from the substrate such as by, for example, usinga hydrofluoric acid etching chemistry so as to provide the structure asshown in FIG. 5.

In FIG. 6, a metal paste 24 is stamped into the openings. In thisapplication, the stamp includes projections that are narrower than theopenings 24 in which it is stamped. That is, the stamp projection widthand/or diameter is less than the width and/or diameter of the opening.The stamp is dipped into the metal paste and subsequently inserted intothe opening 24, thereby transferring the metal paste into the opening.The viscosity of the metal paste 24 is selected so as to permit theopening 22 to be filled with the metal paste upon removal of the stampfrom the opening. In one embodiment, the metal paste is aluminum based,which permits the use of lower annealing temperatures relative to silverbased metal pastes. For example, a suitable metal paste is an AlNi basedpaste, which effectively lowers the annealing temperature to less than400° C.

The aluminum based metal paste 24 is then annealed at a low temperatureto form an annealed metal 26 as shown in FIG. 7. The annealed metalsecures good contact at low resistance to the n-doped silicon surface 14to provide the photovoltaic cell with contact grid and bus bars. In oneembodiment, the annealing temperature is less than 400° C. to greaterthan 300° C. for a period of 0.5 h; in another embodiment, the annealingtemperature is less than 380° C. to greater than 320° C. for a period of0.4 h; and in still other embodiments the annealing temperature is lessthan 360° C. to greater than 340° C. for a period of 0.25 h. By way ofexample, the annealing temperature can be 400° C. for 0.2 h.

Alternatively, the metal paste stamping step can occur prior to removalof the UV cured photoresist and/or polymer 20 shown in FIG. 5. In thisembodiment, the removal of the polymer layer would take place aftermetal stamping and annealing. Advantageously, this helps to support thecleanliness throughout the process.

FIG. 8 provides a schematic of the process flow 100. Photovoltaic cellshaving an antireflective coating are provided in step 110. These cellsare pattern stamped using soft stamp technology as shown in step 120.The transferred photoresist and/or polymer is then UV cured as shown instep 130. In step 140, a wet etching process is used to form openings infor the contract grid in the dielectric layer. The polymer is then stripas shown in step 150 and an aluminum based metal paste is stamped intothe openings as shown in step 160. In step 170, the aluminum based metalpaste is annealed at a relatively low temperature.

Advantageously, the process as described herein is relatively simple anduses minimal capital equipment. The stamping material has a low cost andstamp alignment is generally not an issue since large dimensions areused. The line width, dimension of highest accuracy, to be stamped is at≧100 microns where the alignment can be easy realized through simplemechanical fixture holding the soft stamp as well as the solar cell. Theannealing temperature is below that where any degradation occurs withinthe photovoltaic cell. Still further, the process can be established atexisting silicon based photovoltaic fabrication facilities without anysignificant investment and production costs.

The following examples are presented for illustrative purposes only, andare not intended to limit the scope of the invention. Efforts have beenmade to ensure accuracy with respect to numbers (e.g., amounts,temperature, etc.), but allowance should be made for the possibility oferrors and deviations

EXAMPLE 1

In this example, a comparison was made of a typical prior art processthat included screen printing, paste drying, and firing to form thecontract grid to a process in accordance with the disclosure thatincluded stamping the desired contact grid pattern onto a substrate,curing the UV sensitive polymer, etching the stamped pattern, and anannealing step to form the contact grid.

Process step Process time Process step Process time (prior art) (hours)(new) (hours) Mask apply and 0.25 Stamping and 0.25 screen printingcuring Paste drying 0.25 Etch and clean 0.5 Firing at 900° C. 1.5Stamping, anneal 0.75 at 400° C. and strip/clean

The lead time saving was about 0.5 hours, which is between 15% and 20%of the process lead time. Also, the degradation was less due to thelower annealing temperature used (900° C.˜400° C.). Moreover, withregard to the prior art process, the bulk life time also decreasedbecause the annealing temperature was above 750° C.

While the preferred embodiment to the invention has been described, itwill be understood that those skilled in the art, both now and in thefuture, may make various improvements and enhancements which fall withinthe scope of the claims which follow. These claims should be construedto maintain the proper protection for the invention first described.

1. A process for fabricating a contact grid for a photovoltaic cell, theprocess comprising: providing a photovoltaic cell having anantireflective coating disposed on a sun facing side, the photovoltaiccell comprising a silicon substrate having a p-n junction; soft stampinga pattern of a UV sensitive photoresist and/or polymer onto theantireflective coating; exposing the UV sensitive photoresist and/orpolymer to ultraviolet radiation to cure the UV sensitive photoresistand/or polymer; etching the pattern to form openings in theantireflective coating that define the contact grid; stripping the UVsensitive photoresist and/or polymer; and depositing a conductive metalinto the openings defined by the pattern.
 2. The process of claim 1,wherein the contact grid includes bus bars.
 3. The process of claim 1,wherein etching the pattern comprises a phosphoric acid based wetetching chemistry.
 4. The process of claim 1, wherein stripping the UVsensitive photoresist and/or polymer comprises exposing the UV sensitivephotoresist and/or polymer to hydrofluoric acid.
 5. The process of claim1, wherein depositing the conductive metal comprises stamping metalpaste into the pattern; and annealing the metal paste.
 6. The process ofclaim 5, wherein annealing the metal paste is at a temperature of lessthan 400° C.
 7. The process of claim 5, wherein the metal paste isaluminum based.
 8. The process of claim 5, wherein stamping the metalpaste into the openings of the pattern comprises dipping a stamp intothe metal paste, wherein the stamp comprises a plurality of projectionscorresponding to the openings having a diameter and/or width smallerthan the openings defined by the pattern; inserting the projections intothe openings; and filling the openings with the metal paste.
 9. Theprocess of claim 8, wherein stamping the metal paste into the openingsof the pattern is subsequent to exposing the UV sensitive photoresistand/or polymer to ultraviolet radiation to cure the UV sensitivephotoresist and/or polymer and etching the pattern in the antireflectivecoating to form the openings and prior to stripping the UV sensitivephotoresist and/or polymer.
 10. A process for fabricating a contact gridfor a photovoltaic cell, the process comprising: providing aphotovoltaic cell having an antireflective coating disposed on a sunfacing side, the photovoltaic cell comprising a silicon substrate havinga p-n junction; stamping a pattern of a photoresist and/or polymer ontothe antireflective coating; curing the photoresist and/or polymer;etching the pattern in the antireflective coating to form openings thatdefine the contact grid; stripping the UV sensitive photoresist and/orpolymer; stamping an aluminum based metal paste into the openings of thepattern by dipping a stamp into the aluminum based metal paste, whereinthe stamp comprises a plurality of projections corresponding to theopenings having a diameter and/or width smaller than the openings;stamping the projections into the openings; and filling the openingswith the metal paste; and annealing the aluminum based metal paste. 11.The process of claim 10, wherein the contact grid includes bus bars. 12.The process of claim 10, wherein etching the pattern comprises aphosphoric acid based wet etching chemistry.
 13. The process of claim10, wherein stripping the UV sensitive photoresist and/or polymer fromthe antireflective coating comprises exposing the UV sensitivephotoresist and/or polymer to hydrofluoric acid.
 14. The process ofclaim 10, wherein annealing the aluminum based metal paste comprisesheating to at a temperature of less than 400° C.
 15. The process ofclaim 10, wherein stamping the aluminum based metal paste into theopenings of the pattern is subsequent to curing the UV sensitivephotoresist and/or polymer and etching the pattern in the antireflectivecoating to form the openings and prior to stripping the UV sensitivephotoresist and/or polymer.